Conventionally, an image processing apparatus such as a digital copying machine and a facsimile processes a document composed of a character region, a photographic region, or a halftone dot region, or a document composed of a mixture of such regions. The image processing apparatus reads a document including such regions using an image sensor such as CCD (Charge Couples Device), and converts the document thus read into an image signal.
Further, the image processing apparatus carries out image processing so as to improve the image quality of a recorded image obtained from the image signal. For example, when the image is characters, an enhancement process is carried out to enhance the edges of the image, and when the image is halftone dots, a smoothing process is carried out so as to prevent moire. Also, when the image is characters, it is a common practice to carry out a process for converting the image into a character code.
In either process, it is often the case that an appropriate image process differs for each of the different types of an image, and in order to improve the image quality of the recorded image, the image process needs to be carried out in accordance with the type of the image.
As a conventional technique for improving the image quality using a filtering process, for example, the image processing apparatus as disclosed in Japanese Examined Patent publication No. 21384/1993 (Tokukohei 5-21384) is available. In this image processing apparatus, an image signal which has been smoothed and an image signal which has been enhanced are combined or selected by the output of the image signal from edge component detecting means so as to be outputted.
Also, for example, in a filter processing device as disclosed in Japanese Unexamined Patent publication No. 246076/1988 (Tokukaisho 63-246076), when the edge component of the image signal is not extracted by edge extracting means, a signal which has been subjected to a filtering process for removing a halftone dot component is selected, and when the edge component is extracted, a signal which has not been subjected to a filtering process is selected.
However, discrimination of image types by detection of the edge component alone is not sufficient, and there is a need for discriminating whether the image belong to a character region, a photographic region, or a halftone dot region, and to carry out an image process in accordance with the result of discrimination.
As an image discrimination method for improving the image quality in the described manner, a method in which an image is segmented into blocks, each composed of a plurality of pixels, so as to discriminate image types per each block has been adopted widely. Here, when the image types are to be discriminated per each block by pattern matching, in order to compare the image of each block with one another, it is required to prepare a large number of patterns beforehand. As a result, the memory capacity for storing the patterns is increased, and this method is not suitable for practical applications.
Thus, in these days, it has been a common practice to adopt a method in which feature parameters are extracted from each block by a predetermined procedure and image types are discriminated in accordance with the feature parameters thus extracted.
For example, in the halftone dot-photographic region discrimination method as disclosed in Japanese Unexamined Patent publication No. 194968/1986 (Tokukaisho 61-194968), in a certain block, a change in signal level between two succeeding pixels in a main scanning direction is measured, and the sum of measured values in the block is calculated. In the same manner, the sum of the change in signal level is also calculated with respect to a sub scanning direction. The sums of measured values in the main scanning direction and in the sub scanning direction are respectively compared with a predetermined set value, and the image type of each block is discriminated in accordance with the result of this comparison.
As another method employing feature parameters, for example, Japanese Unexamined Patent publication No. 147860/1987 (Tokukaisho 62-147860) discloses a half-tone facsimile signal processing method. In this method, a difference between a maximum signal level and a minimum signal level of a certain block are determined, and the difference thus determined is compared with a predetermined first set value. When the difference is smaller than the first set value, since the block is a portion in which a change in signal level is gradual in this case, the block is judged to be a photographic region. On the other hand, when the difference is larger tan the first set value, since the block is a portion in which a change in signal level is abrupt in this case, the block is judged to include a contour of characters and pictures, or halftone dot-photographic portion.
Also, in this method, it is judged whether there is a change in signal level between two pixels succeeding specially in a block in accordance with an access order which has been set beforehand so as to calculate the number of changes. The number of changes thus calculated is compared with a second set value which has been set beforehand, and when the number of changes is larger than the second set value, the block is judged to be a halftone dot region. On the other hand, when the number of changes is smaller than the second set value, the block is judged to be a region other than the halftone dot region. The image type of each block is judged in accordance with the judgement based on the level difference and the judgement based on the number of changes.
However, in the conventional image discrimination methods as described above, the discrimination accuracy is not sufficient, and misjudgment of image type is likely to occur. Thus, in order to realize higher image quality, further improvement in the discrimination accuracy is needed.
Here, the lowering of discrimination accuracy is caused generally by unmatching of feature parameters, namely, the characteristics of the character region, the photographic region, and the halftone dot region are not sufficiently represented by feature parameters. Also, misjudgment is induced by the following important factors of (a) a method for categorizing image types based on feature parameters is inappropriate and (b) the threshold values (set values mentioned above) for categorizing image types are not selected appropriately.
Specifically, in the described prior art, a predetermined threshold value is set for each of the feature parameters. Also, each of the feature parameters and each threshold value are individually compared, and after carrying out judgement per feature parameter, the blocks including pixels are categorized in accordance with the result of each judgement. For this reason, it is difficult to set an appropriate threshold value for each feature parameter, and even when the number of feature parameters is increased, it is difficult to obtain sufficient discrimination accuracy.
Also, in the described prior art, the type of region is judged per each block. Therefore, when the region type is misjudged, the entire pixels of the block are affected, and the image quality is greatly lowered. Furthermore, because the feature parameters of the target pixel reflect only limited image processing characteristics, an optimum image process cannot be carried out in accordance with each target pixel.
As a countermeasure against these problems, Japanese Unexamined Patent publication No. 125857/1996 (Tokukaihei 8-125857) discloses an image processing apparatus in which the region type of a block is judged in accordance with a combination of plural feature parameters, and the region type of surrounding blocks are judged per each pixel. This prevents the discrimination accuracy from being lowered by inappropriate selection of the threshold values and inappropriate categorizing method.
However, even in an image processing apparatus having this arrangement, the characteristic of each region type is not fully represented by the feature parameters, and further improvement in discrimination accuracy is needed.